Aluminium alloy sheet with roughened surface

ABSTRACT

There is disclosed a process for producing an aluminium alloy sheet, which comprises subjecting a surface of the sheet to anodising conditions to form on the surface an aluminium oxide barrier layer of thickness 10 to 50 nm and treating the oxide layer with an aqueous solution of alkali to remove the layer, thereby leaving a roughened surface on the alloy sheet.

[0001] The present invention relates to an aluminium alloy sheetmaterial having a roughened surface. It also relates to a process forproducing such a sheet material and to the use of such sheet materials.Such sheet materials are of particular use in the production oflithographic plates.

[0002] At present the lithographic sheet market largely consists ofproducts in the AA1XXX and AA3XXX alloy range. An alloy sheet, beingprepared for use as a lithographic material, is conventionally cleanedby the metal producer to remove excess oil, oxide and metal fines. Thecleaned sheet is then usually chemically etched for a short time,typically 5 to 10 s, in alkali immediately prior to electrochemicaletching (electrograining) in nitric or hydrochloric acid electrolytes bythe plate manufacturer. The action of the pre-etch (chemical etch)removes any naturally-formed oxides on the surface of the alloy sheet toproduce a fresh active aluminium surface which can then be roughened byelectrograining. The fresh aluminium surface obtained by theconventional pre-etch procedure is relatively smooth and contains onlyshallow micropitting.

[0003] To make an aluminium sheet suitable for use as a lithographicplate support its surface needs to be roughened or grained in order toenhance the adhesion of an organic coating on the support, and toimprove the water-retention properties. Application to the support of aphotosensitive layer followed by irradiation and development generallyresults in a lithographic plate having ink-receptive image areas, whichcarry an organic coating, and water retaining non-image areas, thelatter generally being the uncovered support surface. For this purposethe aluminium alloy sheet needs to be roughened on a scale of about Ra 1to 2 μm as measured by an optical non-contact profilometer. Thisroughening is usually, though not necessarily, accomplished byelectrograining. The present invention provides pre-anodising thenetching prior to the steps of the standard graining process.

[0004] The cost of the graining or roughening step is an important partof the economics of lithographic plate support manufacture. In oneaspect, the present invention is based on our discovery that rougheningof an aluminium alloy surface can be achieved more economically than isachieved by the usual method involving a conventional pre-etch step.

[0005] Nitric acid electrograining is very susceptible to surfacedefects which can manifest themselves on the final lithographic printingplate. One such defect is known as non-etch defect. This appears as abright ungrained streak which typically can be about 100 μm wide andseveral millimetres long. It is generally accepted in the art that thecause of non-etch defect is passivation during the electrograiningprocess at the alloy surface. Passivation can be caused by localcontaminating films, rolled in metal, or rolled in noble particles, e.g.of copper-rich material which cause local passivation. We have foundthat non-etch defects can be overcome or at least reduced by the presentinvention.

[0006] GB-A-2145738 discloses a process for anodising aluminium foil forelectrolytic capacitors. A boehmite type film is produced and the foilis anodised in a phosphate electrolyte. The final foil has an anodicfilm which is, therefore, not totally removed.

[0007] EP-A-0645260 discloses a method of producing a support for aplanographic printing plate comprising electrochemical roughening of analuminium plate and etching with an alkali. No anodising step isdisclosed.

[0008] U.S. Pat. No. 6,024,858 discloses a process for producing analuminium support comprising chemical etching and electrochemicalroughening. Anodising in an acidic solution is also disclosed butremoval of the anodic film is not disclosed.

[0009] U.S. Pat. No. 5,731,124 discloses a method for preparing analuminium foil comprising roughening and subsequent anodising.Subsequent washing with sodium bicarbonate is taught.

[0010] U.S. Pat. No. 5,556,531 discloses a process for the treatment ofaluminium materials comprising treating an aluminium oxide layer with asolution of an alkali metal silicate and rinsing the treated layer. Sucha treatment does not remove the oxide layer.

[0011] U.S. Pat. No. 5,282,952 discloses a method for preparing asubstrate for lithographic printing plates. The process includes thestep of anodising a plate, but there is not complete removal of theoxide layer.

[0012] U.S. Pat. No. 5,104,484 discloses a method for manufacturing asubstrate for presensitised plates comprising electrolytic rougheningand etching with an alkali or acid. The plate is anodised as a finalstep and the resulting film is not removed.

[0013] U.S. Pat. No. 4,980,271 discloses developer compositions forlithographic printing plates. Standard methods of preparing a plate bychemical or electrochemical graining and anodising are disclosed.

[0014] U.S. Pat. No. 4,689,272 discloses a treatment of aluminium oxidelayers comprising treatment with an aqueous alkali and treatment of aseparated oxide layer with an aqueous solution containing an organicpolymer.

[0015] U.S. Pat. No. 4,545,866 discloses a modified electrograiningprocess which includes a final conventional anodising step.

[0016] U.S. Pat. No. 4,492,616 discloses a process for treatingaluminium oxide layers where an anodising step is the final step. Theresulting layer is conditioned, but not removed.

[0017] U.S. Pat. No. 4,483,913 discloses a planographic printing plate.The anodic film thereon is conditioned, but not removed.

[0018] DE-A-3717757 discloses the production of a substrate for making alithographic printing plate including graining, anodising andhydrophilising.

[0019] DE-A-3335440 discloses a process in which etching is carried outafter applying a photochemical layer to the anodised plate.

[0020] Therefore, the methods disclosed in the prior art often discloseelectrograining without pre-anodising and, where an anodic film isproduced, it is not dissolved completely.

[0021] According to a first aspect, the present invention provides aprocess for producing aluminium alloy sheet having a roughened surfacewhich process comprises the steps of (1) subjecting a surface of thealuminium alloy sheet to be treated to anodising conditions to form onthe said surface an aluminium oxide barrier layer having a barrier layerthickness in the range of from 10 to 50 nm, and (2) treating thealuminium oxide barrier layer with an aqueous solution of alkali at atemperature of 35° C. to 80° C. for a period of time sufficient toremove entirely or substantially entirely said layer from the alloysurface thereby leaving a roughened surface on the alloy sheet.

[0022] If the layer is not removed entirely or substantially entirelyfrom the surface, poor graining may result. Entire removal of the layeris preferred.

[0023] We have found that by anodising the alloy under controlledconditions to form an aluminium oxide barrier layer on the surface ofthe alloy sheet and then subjecting this to a chemical etch in aqueousalkali solution an alloy surface is produced which is pitted androughened to a greater degree than a surface not previously providedwith such an anodic barrier layer.

[0024] Although we do not wish to be bound by theory we believe that inthe early stages of etching, the alkali etch attacks flaws in the anodicoxide layer with continued dissolution to the aluminium surface. Oncontact with the aluminium surface dissolution of the aluminium occursunder the anodic oxide layer on the alloy surface. Effectively, a ‘well’is produced between the metal surface and the anodic oxide layer wherelocal dissolution of the aluminium surface causes a relatively deep pit.Also, at the same time there is dissolution of the oxide layer and aftera period of time this layer completely dissolves to leave a deeplymicropitted, or roughened, surface on the alloy sheet. It will beappreciated that a process that comprises a chemical etch step whichproduces a surface of the alloy sheet which is more micropitted, orroughened, compared to a conventional etch has the advantage that thesubsequent electrograining can be carried out for a shorter time periodthan is used conventionally.

[0025] The roughened surface of an aluminium sheet prepared by theprocess of the invention can then be treated in the conventional way,including the step of applying a photosensitive layer followed byirradiation and development, for use as a lithographic sheet.

[0026] According to the present invention the aluminium alloy sheet isanodised to form, on its surface, an aluminium oxide barrier layer. Thealuminium alloy will preferably be one selected from the AA1XXX or theAA3XXX alloy series. Examples of alloys that may be used in the presentinvention include AA1050A alloys, AA1200A alloys and AA3103 alloys whichare preferred for lithographic use. In view of its better properties,particularly Its good graining response, AA1050A alloy is mostpreferred.

[0027] Typically, the aluminium alloy sheet surface will be cleaned andanodised using phosphoric acid electrolyte to provide an aluminium oxidebarrier layer having a layer thickness in the range of from 10 to 50 nm.By the term “aluminium oxide barrier layer” we mean an oxide layer whichhas barrier properties on the surface of the aluminium alloy sheet.Preferably the layer will be non-porous. However, the layer may containsome pores provided that these do not compromise the barrier propertiesof the oxide layer. A porous oxide layer, which does not provide barrierproperties, does not solve the technical problem which is solved by theuse of an oxide layer having barrier properties.

[0028] The anodising procedure used in the process of the invention mayuse either direct current (d.c.) current or, more preferably,alternating current (a.c.). The a.c. waveform may be sinusoidal or notas desired. The a.c. current may be biased in either the cathodic oranodic direction. The a.c. frequency is at least several cycles persecond and is, preferably, the commercial frequency.

[0029] The electrolyte concentration, the aluminium level of theelectrolyte, the temperature, the current density and the time ofanodisation all, of course, affect the thickness of the oxide layerproduced. Typically, the phosphoric acid concentration will be withinthe range of 10 to 30%, with approximately 20% phosphoric acid beingpreferred in order to obtain a good compromise of the processparameters. It should be noted that other acids may be used to achievethe same effect, typical examples being nitric acid, sulphuric acid, orother phosphorus-containing acids. The electrolyte will containaluminium typically up to about 20 g/l and preferably at a concentrationin the range of from 3 to 15 g/l. The anodising treatment will typicallybe carried out using a solution at an elevated temperature, andtypically at a temperature in the range of from 40° C. to 80° C.,preferably 45° C. to 70° C. Anodcising will typically be carried putusing a current density of 1 to 5 kAm⁻², preferably 2 to 3 kAm⁻². Theanodising treatment-will typically be carried out for up to severalseconds in order to produce an oxide layer having the desired layerthickness. We have found that oxide layers having a thickness range offrom 10 to 28 nm can be produced by anodising in 20% phosphoric acidcontaining 3 to 15 gl⁻¹ aluminium temperatures in the range of from 55°C. to 80° C. and current densities of from 2 to 3 kAm⁻² for about 0.5 s.By way of example, the barrier films shown in the following Table 1 maybe prepared by anodising AA1050A alloy sheet in 20% phosphoric acidcontaining about 8 gl⁻¹ aluminium. TABLE 1 Temperature Current DensityBarrier ° C. kAm⁻² Film nm 55 3 28 60 3 31 65 3 28 70 3 26 70 2 18 75 217 80 2 10

[0030] After the anodising stage the anodised aluminium alloy sheet maybe stored or treated without substantial delay to the etching step. Onthe grounds that the anodised alloy sheet may, itself, be an item ofcommerce and that a sheet having a non-porous aluminium oxide layerhaving a barrier layer thickness in the range of from 10 to 50 nm,preferably 10 to 30 nm, for example 20 to 30 nm can be used to providespecial benefits described herein the invention in a further aspectprovides a sheet of aluminium alloy having on a surface thereof anon-porous aluminium oxide layer having a barrier layer thickness in therange of 10 to 50 nm, preferably 10 to 30 nm, for example 20 to 30 nm.The aluminium alloy is preferably one selected from AA1XXX and AA3XXXalloys as described above and most preferably AA1050A alloy. Theinvention according to yet a further aspect provides the use of a sheetof aluminium alloy having on a surface thereof a non-porous aluminiumoxide layer having a barrier layer thickness in the range of from 20 to30 nm in the manufacture of aluminium alloy sheet having a roughenedsurface which manufacture comprises treating the non-porous aluminiumoxide layer with an aqueous solution of alkali at a temperature in therange of from 35° C. to 80° C. for a period of time sufficient to removeentirely or substantially entirely said layer from the alloy surfacethereby leaving a roughened surface on the alloy sheet.

[0031] The anodised aluminium alloy sheet is then treated to a chemicaletch using an aqueous solution of alkali of a temperature in the rangeof from 35° C. to 80° C. for a period of time sufficient to removeentirely or substantially entirely the aluminium oxide layer from thealloy surface thereby leaving a deeply micropitted, or roughened,surface on the alloy sheet.

[0032] The aqueous solution of the alkali, typically NaOH or KOH,preferably has a concentration by weight of from 1 to 10%. Mostpreferably, the alkali used in the etching step is 2 to 5% NaOH. Thechemical etch will be carried out for a period of time sufficient tocause the complete removal of the aluminium oxide layer at which pointthe alloy surface will be roughened. Surfaces have been etched in sodiumhydroxide solutions for varying amounts of time. The degree ofmicropitting obtained has been shown to depend on the etch time in thesodium hydroxide and on the thickness of the aluminium oxide layer onthe anodised alloy sheet. The degree of micropitting increases withsodium hydroxide etch time to a point of maximum roughness and minimumgloss. This also depends on the barrier layer thickness. For example theroughness of the surface with a 20 nm barrier layer reaches its maximumand reaches lowest gloss after about 3 to 5 seconds in 3% NaOH at 60° C.A similar surface with 28 nm of barrier layer reaches a maximumroughness and lowest gloss after about 8 seconds. A surface with nobarrier layer shows relatively smooth surfaces when similarly treatedwith no evidence of the type of micropits formed when a barrier layer ispresent on the surface of aluminium. Alkali etching past the point oflowest gloss begins to overcome any of the benefits described until,eventually, a surface similar to that obtained by etching a non-anodisedstarting material is obtained. The pitted surface is effectivelysmoothed by further alkali etching.

[0033] As mentioned above, the process of the invention which provides aroughened surface on aluminium alloy sheet gives rise to advantages whenthe sheet is subjected to electrograining according to conventionaltechniques. Firstly, because the alloy sheet produced according to theprocess described above has a surface which has greater roughnesscompared to standard treated alloy sheet a subsequent step ofelectrograining can be carried out for a shorter period of time(compared to the conventional technique) to provide a surface on thealloy having satisfactory graining. This shorter electrograining timeprovides a reduced consumption of chemicals and less costly waste.Accordingly, the present invention further provides a method of makingan electrograined aluminium alloy sheet which comprises (1) subjecting asurface of a sheet of an aluminium alloy, preferably selected fromAA1XXX and AA3XXX alloys, to anodising conditions to form on the saidsurface an aluminium oxide barrier layer having a barrier layerthickness of from 10 to 50 nm; (2) treating the aluminium oxide layerwith an aqueous solution of alkali at a temperature of 35° C. to 80° C.for a period of time sufficient to remove entirely or substantiallyentirely said layer from the alloy surface thereby leaving a roughenedsurface on the alloy sheet, and (3) subjecting the roughened surface ofthe alloy sheet to electrograining.

[0034] Although electrograining may be carried out using nitric acid orhydrochloric acid in the present invention it is preferred that nitricacid electrograining is used. The conditions employed for theelectrograining step of the process are those that are known in the art.

[0035] In the case of nitric acid electrograining the present inventionalso provides a means of reducing non-etch defects. As reported hereinnon-etch defects can be substantially reduced by use of an alloy sheet,as the feedstock for the nitric acid electrograining procedure,roughened by the process described herein.

[0036] According to a further aspect of the present invention, there isprovided an aluminium sheet formed by the process of the invention foruse as a lithographic substrate. Thus, aluminium alloy sheets roughenedin accordance with the process of the invention may advantageously beused as substrates for additive grained plates. Additive graining is aprocedure whereby at least one coating is applied to a cleaned androlled surface to give the desired wear, hydrophilic and adhesionproperties of the lithographic substrate. Such coatings may betransparent so a uniform appearance to the substrate is desirable bothaesthetically and for operational purposes. The micropitted surfacesobtained by the invention will promote bonding to coatings applied tothe alloy sheets and give a more uniform appearance to the substrate.Examples of such coatings include sol gel coatings or films thatcomprise a hydrophilic and a hydrophobic layer where the top layer canbe removed, for example, by thermal ablation.

[0037] Furthermore, other uses of the aluminium sheet are envisaged, forexample canstock, auto sheet, reflector sheet etc.

EXAMPLES Example 1

[0038] Samples of AA1050A lithographic sheet were anodised using 20%phosphoric acid containing approximately 8 g/l Al for 0.5 s with thefilm thickness variation achieved by varying the a.c. current densityand treatment temperature as shown in Table 2. TABLE 2 Temperature ° C.2 kAm⁻² 3 kAm⁻² 55 28 nm (D) 60 31 nm 65 28 nm 70 18 nm (B) 26 nm (C) 7517 nm 80 10 nm (A)

[0039] Th samples prepared above and as shown in Table 2 were treatedwith 3% NaOH at 60° C. for 10 s. For comparison a sample of AA1050Alithographic sheet similar to those anodised above was cleaned inphosphoric acid, but was not provided with an anodised layer and wastreated to etching using the same etch conditions as were used for theother samples. The 60° gloss values of the treated surfaces weremeasured across the rolling direction using a Rhopoint glossmeter. Thevalues are shown below in Table 3. TABLE 3 Oxide film nm 0 10 (A) 17 18(B) 26 (C) 28 (D) 31 28 60° 429 401 332 318 167 149 147 157 Gloss

[0040] As can be seen from Table 3, the samples having thicker aluminiumoxide barrier layers gave, after the alkali etch, surfaces with lowestgloss values. Scanning Electron Microscopy (SEM) showed these alloysurfaces to be deeply micropitted when compared to the standard cleanedalloy surface (no anodised layer) after a similar alkali etch.

Example 2

[0041] A sample of lithographic alloy sheet AA1050A was cleaned in 20%phosphoric acid and then subjected to an alkali etch. Further samples ofthe same alloy sheet material were anodised as in Example 1 to producenon-porous aluminium oxide films of thickness 10, 20 and 28 nm. Thesewere also subjected to an alkali etch. The alkali etch in all cases wascarried out using 3% NaOH at 60° C. for up to 20 s. The gloss values ofthe alkali etched surfaces were measured, using a Rhopoint glossmeter,before etching and after etch times of 1, 2, 3, 4, 5, 8, 10, 15 and 20seconds. Plots of the relationship between etch time and 600 gloss valuefor the non-anodised sample (PTL std) and for the anodised samples areshown in FIG. 1. A non-anodised sample that had been commerciallycleaned electrolytically in phosphoric acid was included for comparisonpurposes (PTL std).

[0042] From FIG. 1 it can be seen that the level of gloss achieveddepends on the thickness of the non-porous aluminium oxide layer and thedegree of alkali etching. A thin anodised layer of 10 nm gives a similarresponse to the cleaned but not-anodised material. However, the samplehaving an aluminium oxide layer of 20 nm thickness reached a minimum 600gloss value after 3-5 s etch time whereas the sample having an aluminiumoxide layer of 28 nm reached a minimum 60° gloss value after about 8 sof etch time. The cleaned but non-anodised sample shows a relativelysmooth surface throughout the etch period with no evidence of the typeof, and degree of, micropitting obtained for samples originally providedwith anodised layers.

[0043] It is clear from the results obtained that the degree ofmicropitting increases with etch time to a point of maximum roughnessand minimum gloss. Alkali etching past the point of lowest gloss beginsto reverse the benefits obtained until, eventually, a surface similar tothat obtained with the non-anodised material is reached. The pittedsurface is, thus, effectively smoothed by further alkali etching beyondthe minimum gloss point.

[0044] The average roughness Ra of the surfaces of the samples at theirminimum 60° gloss values achieved by etch was determined using a PerthenFocodyn or LSI probe. These values are shown below in Table 4. TABLE 4Thickness of Minimum 60° Average Roughness Alkali Etch Time Anodic GlossValue Ra at Minimum to Minimum Layer (nm) Achieved Gloss (μm) Gloss (s)0 400-450 0.35-0.40 0-20 10 400-450 0.35-0.40 0-20 20 330-350 0.45-0.503-5  28 160-180 0.45-0.52 8-10

Example 3

[0045] Experiments were carried out to investigate the time taken toachieve a satisfactory grained surface by electrograining samples oflithographic sheet (AA1050A).

[0046] Conventionally, before electrograining, lithographic sheet istypically etched in an alkali solution, such as 3% NaOH for 10 secondsat 60° C. This gives an etched surface with average roughness Ra between0.35 and 0.4 microns and 60° gloss value of between 400 and 450. Thepurpose of this treatment is to remove a small amount of the surface andto activate it prior to electrograining. If this is not done, anunsatisfactory grained surface is produced.

[0047] I n this example we subjected samples of 1050A sheet toelectrograining in nitric acid. The samples were grained in a laboratorytwin cell system the liquid contact mode. The electrolyte was 1.5%nitric acid. The voltage applied was 14V a.c. (conventional sine wavesource). The spacing between each electrode was 15 mm and the counterelectrodes were conventional impregnated graphite used industrially. Dueto the symmetrical nature of the arrangement the forward and reversecurrent density is approximately equal. The samples were:

[0048] 1. AA1050A sheet precleaned in phosphoric acid and then etchedfor 10 s in 3% NaOH at 60° C.;

[0049] 2. AA1050A sheet anodised according to the procedure described inExample 1 to produce a non-porous aluminium oxide layer having athickness of 20 nm. This was then etched in 3% NaOH at 60° C. for 10 s;and

[0050] 3. AA1050A sheet anodised according to the procedure described inExample 1 to produce a non-porous aluminium oxide layer having athickness of 28 nm. This was then etched in 3% NaOH at 60° C. for 10 s.

[0051] In each case the surface of the sample was electrograined in 1.5%nitric acid at 40° C. and at 50 Adm⁻² for 10 s and the average roughness(Ra) and the 600 gloss value of the electrograined surface weredetermined as in Example 2. This procedure was repeated forelectrograining times of 13, 15 and 18 s. The average roughness valuesof the samples after electrograining are shown in Table 5 and the 60°gloss values are shown below in Table 6. TABLE 5 Ra Values of SurfacesAfter Different Electrograining Times Anodic Film Thickness Prior 10 1315 18 to Alkali Etch Second Second Second Second  0 0.776 0.89 0.9460.962 20 nm film 0.893 0.999 0.985 0.981 28 nm film 0.846 0.924 0.9411.036

[0052] TABLE 6 60° Gloss Values of Surfaces After DifferentElectrograining Times Anodic Film Thickness Prior 10 13 15 18 to AlkaliEtch Second Second Second Second Standard 2.2 1.7 1.6 1.5 20 nm film 1.81.5 1.4 1.3 28 nm film 1.7 1.4 1.4 1.1

[0053] From these results it can be seen that surfaces with similargloss and roughness can be obtained more quickly using etchedpre-anodised surfaces having anodic layer thickness of 20 or 28 nm.

Example 4

[0054] Further electrograining experiments in the microcell systemdescribed in Example 3 with a range of electrograining conditions alsoshowed electrograined surfaces can be obtained faster with pre-anodisedsurfaces. The sodium hydroxide pre-etch is as described above in Example3. The results are shown below in Table 7. TABLE 7 14 V 13 V 12 V 11 VAnodic Layer Electrograining Time (s) Thickness 15 20 25 30 16 22 27 3318 24 30 36 20 26 33 39  0 X / ◯ ◯ X / ◯ ◯ X / ◯ ◯ X X ◯ ◯ 10 nm X / ◯ ◯X / ◯ ◯ X / ◯ ◯ X / ◯ ◯ 20 nm X / ◯ ◯ X ◯ ◯ ◯ X ◯ ◯ ◯ X / ◯ ◯ 26 nm X ◯◯ ◯ X ◯ ◯ ◯ X ◯ ◯ ◯ X / ◯ ◯ 28 nm X ◯ ◯ ◯ X ◯ ◯ ◯ X ◯ ◯ ◯ X ◯ ◯ ◯

[0055] Normal electrograining in the microcell is considered to be 14volts 30 seconds where a visually good structure is formed with therequired pit morphology and roughness. Visually good graining is seen asearly as 25 seconds. However, previous work on electrograiningsimulations have shown anything slightly less than 25 seconds can leavethe surface slightly undergrained with insufficient coverage androughness and the roll lines are still visible and too close to theborderline of the process. Therefore electrograining for 30 secondsovercomes any process variations from the alloy chemistry or any slightvariations in the cleaning and electrograining process. The above tableillustrates this and that even after 25 seconds the surface is visuallygood with all material including standard AA1050A. The above table showsthat good surfaces can be obtained faster at certain graining conditionswith pre-anodised surfaces and that benefits were apparent with a rangeof thickness between 10 and 30 nm. However, it is clear that fewerbenefits were found with alloy sheet samples originally provided with 10nm anodic film or less (before etch) and more benefits were found withthe 28 nm anodic film alloy sheet.

Example 5

[0056] Samples from a ‘worst ever’ non-etch defect coil of AA1050A sheetwere prepared as for anodising conditions A, B, C and D as described inExample 1. These were then etched in 3% NaOH for 10 s at 60° C. andelectrograined in a laboratory tank in 1.5% nitric acid at 50 Adm⁻² at40° C. for 13 s. The number of non-etch defects (known as stege) werecounted and the results are shown in FIGS. 2A, B, C and D. In each casethe results are also shown for a non-etch defect count taken on thestandard material, i.e. un-anodised (PTL std) material but subjected tothe same etch and electrograining procedures described above for theanodised materials.

[0057] As can be seen from the Figures, the number of non-etch defectswas seen to decrease by about 40% with sample A (pre-anodised with 10 nmlayer) and about 70% with samples B, C and D (pre-anodised with 20 nm,26 nm and 28 nm layers, respectively).

Example 6

[0058] Nitric Acid Electrograining Studies of Alkali Etched Pre-AnodisedAA1050A Lithographic Sheet

[0059] Standard PTL cleaned, pre-anodised samples as prepared in B (20nm) and D (28 nm) in Example 1 were etched in 3% NaOH at 60° C. for 2,5, 8, 10, 12 and 15 seconds followed by rinsing in deionised water. Allsamples were electrograined at 50 Adm⁻² in 1.5% nitric acid at 40° C.for 10, 13, 15 and 18 seconds. A 10 second alkali etch and 18 secondselectrograining represents a full treatment with normal commerciallycleaned litho sheet in the laboratory tank. The following 60° C. glossvalues in Table 8 were measured and give an indication of the degree ofgraining. TABLE 8 60° C. Gloss Values of Surfaces After ElectrograiningE/g t std B D E/g t std B D  2s NaOH  5s NaOH  5 20.2 184 250  5 8.613.5 27 10 7.1 90 142 10 4.4 7.6 8.3 13 3.5 21 115 13 2.9 2.5 11 15 2.826 110 15 2.3 1.9 10.3 18 1.9 8.5 64 18 2 3 12.2  8s NaOH 10s NaOH 104.6 5.3 24 10 3.8 3.6 4.3 13 5 2.4 11.3 13 3.6 3.2 2.5 15 2 2 3.2 15 3.22.8 4 18 2 1.9 4.6 18 2.1 1.9 2.3 12s NaOH 15s NaOH 10 5.5 3.6 4.7 104.4 3.7 4.2 13 3.6 1.9 2.3 13 3.4 2.4 2 15 2.1 2 2 15 1.9 2.9 1.9 18 1.92.1 2 18 1.8 2.1 1.8

[0060] From the results above we have plotted the relationship betweenNaOH etch time and 60° gloss values for samples electrograined for 18 s.This is shown in FIG. 3. It is clear from this that the level ofelectrograining is relatively independent of alkali etch time for anormal PTL cleaned lithographic sheet (i.e. non pre-anodised). It isalso evident that a minimum etch time is necessary when electrograiningthe pre-anodised sheet.

[0061] Examination of a similar plot produced for the sampleselectrograined for 13 s (FIG. 4) shows that the pre-anodised surfacesgive lower gloss levels than the PTL standard (non pre-anodised)material for alkali etch times of at least 5 s for B and 10 s for D.They have similar surfaces to normal PTL cleaned material grained for 18s.

[0062] From these results it is possible to obtain the required mattsurface with similar gloss levels faster with pre-anodised surfaces ifthe alkali etch conditions are correct. For example if we choose an 8second etch it may be sufficient for a 20 nm barrier film as the 28 nmfilm shows no benefits and is, in fact, worse than the normal material.If the alkali etch time is increased to 12 seconds this is wherebenefits in graining are seen with both 20 and 28 nm films. A furtherincrease to 15 seconds indicates any benefits associated with the 20 nmfilm are reduced.

Example 7

[0063] 1050A lithographic sheet was electrolytically cleaned in 20%phosphoric acid at 85° C. with a charge density of about 1 kC/m². It wasthen subject to an alkali etch (approximately 3% sodium hydroxide) at60° C. for up to 12 seconds. Further samples of this sameelectrolytically cleaned starting material were anodised to give an 18nm film as shown in sample B in Example 1. These were also subject to analkali etch for up to 12 seconds. A plot of the relationship betweenetch time and 60° gloss are shown in FIG. 5. A non anodised samplecleaned electrolytically in phosphoric acid was included for comparisonpurposes (PTL standard). From FIG. 5 it can be seen the level of glossachieved for the anodised sample depends on alkali etch time. A minimum60° gloss value was achieved between 24 seconds.

[0064] Experiments were then carried out to investigate the time takento achieve a satisfactory grained surface by electrograining samples oflithographic sheet with the above mentioned samples having an 18 nm filmand standard PTL material.

[0065] Conventionally with this test the sheet is etched for 10 secondsin the alkali etch. Standard and anodised material was subject to thistreatment followed by anodising in nitric acid for 14, 16, 18, 21 and 23s representing 60, 70, 80, 90 and 100% graining.

[0066] The samples were visually examined and the anodised samplesgrained at 18 and 21 s were visually comparable to that of standardmaterial grained for 23 seconds. For the same shorter graining times of18 and 21 s, samples of standard material had a more metallic andvariable appearance. Electrograining in batch processes with nitric acidcan give some unevenness in surfaces. The samples produced showedanodised samples to have a faster and more even graining response. Theresults are illustrated in tables 9 and 10 below and gloss values inFIG. 6.

[0067] The same experiments were repeated with a 6 second alkali etchtreatment which is close to the minima time as shown in FIG. 5. Againthe samples were visually examined and the anodised samples grained at18 and 21 s were visually comparable to that of standard materialgrained for 23 seconds. The samples of standard material grained for thesame shorter times had a more metallic and variable appearance. Theresults are illustrated in tables 11 and 12 below and gloss values inFIG. 7. TABLE 9 60° C. Gloss Values for 10 Seconds Alkali Etch WithAnodised Sample Electrograining % Time (s) graining min max mean sd 23100 2.2 2.7 2.5 0.16 21 90 2.3 3.3 2.8 0.35 18 80 2.1 3.7 3.1 0.49 16 703.1 5 3.9 0.68 14 60 2.9 4.5 3.7 0.52

[0068] TABLE 10 60° C. Gloss Values for 10 Seconds Alkali Etch WithStandard Sample Electrograining % Time (s) graining min max mean sd 23100 2.3 3.3 2.7 0.38 21 90 2.3 4.7 3.1 0.75 18 80 3.6 7.3 4.9 1.13 16 703.8 12.9 7.9 3.4 variable graining 14 60 4.2 24.1 13.8 6.8 variablegraining

[0069] TABLE 11 60° C. Gloss Values for 6 Seconds Alkali Etch WithAnodised Sample Electrograining % Time (s) graining min max mean sd 23100 1.8 2.1 1.9 0.12 21 90 1.8 2.2 1.9 0.13 18 80 2.2 4.6 2.8 2.75 16 702.4 8 3.2 1.8 14 60 2.4 12.1 4.8 2.91

[0070] TABLE 12 60° C. Gloss Values for 6 Seconds Alkali Etch WithStandard Sample Electrograining % Time (s) graining Min max mean sd 23100 2.2 2.8 2.4 0.2 21 90 2.3 3 2.7 0.21 18 80 3 12.7 8.1 3.2 variablegraining 16 70 4 9.2 5.8 1.5 variable graining 14 60 3.7 11.5 5.6 2.4variable graining

1. A process for producing aluminium alloy sheet having a roughenedsurface which process comprises the steps of (1) subjecting a surface ofthe aluminium alloy sheet to be treated to anodising conditions to formon the said surface an aluminium oxide barrier layer having a barrierlayer thickness in the range of from 10 to 50 nm, and (2) treating thealuminium oxide layer with an aqueous solution of alkali at temperatureof 35° C. to 80° C. for a period of time sufficient to remove entirelyor substantially entirely said layer from the alloy surface therebyleaving a roughened surface on the alloy sheet.
 2. A process accordingto claim 1, wherein the aluminium alloy sheet is a sheet of alloyselected from AA1XXX and AA3XXX.
 3. A process according to claim 2,wherein the aluminium alloy sheet comprises a sheet of AA1050A, AA1200Aor AA3103 alloy.
 4. A process according to claim 1, wherein in step (1)the aluminium alloy sheet is anodised using 20% phosphoric acidcontaining from 3 to 15 gl⁻¹ aluminium at a temperature in the range offrom 55° C. to 80° C. using a current density in the range of from 2 to3 kAm⁻².
 5. A process according to claim 1, wherein a.c. anodising isused.
 6. A process according to claim 1, wherein d.c. anodising is used.7. A process according to claim 1, wherein the layer thickness of thealuminium oxide barrier layer produced on the surface of the aluminiumalloy sheet is in the range of from 10 to 50 nm.
 8. A process accordingto claim 7, wherein the layer thickness of the aluminium oxide barrierlayer produced on the surface of the aluminium alloy sheet is in therange of from 20 to 30 nm.
 9. A process according to claim 1, whereinthe aluminium oxide barrier layer is non-porous.
 10. A process accordingto claim 1, wherein in step (2) the aqueous solution of alkali isselected from aqueous NaOH and aqueous KOH.
 11. A process according toclaim 10, wherein the aqueous solution of alkali has a concentration inthe range of from 1 to 10%.
 12. A process according to claim 11, whereinthe aqueous solution of alkali is 2 to 5% NaOH.
 13. A process accordingto claim 1, wherein in step (2) the treatment is carried out for aperiod within the range of from 3 to 20 seconds.
 14. A process accordingto claim 1, wherein the aluminium alloy sheet is a sheet of AA1050Aalloy and the surface of the alloy is treated to anodising conditions toform, on the said surface, a non-porous aluminium oxide layer having alayer thickness in the range of from 10 to 30 nm and wherein thealuminium oxide layer is treated, in step (2), with 3% aqueous NaOH at60° C. for from 3 to 8 seconds.
 15. An aluminium sheet formed by theprocess of claim 1 for use as a lithographic substrate.
 16. A sheet ofaluminium alloy having on a surface thereof a non-porous aluminium oxidelayer having a layer thickness in the range of from 20 to 30 nm.
 17. Asheet according to claim 16, wherein the aluminium alloy is AA1050Aalloy.
 18. The use of a sheet of aluminium alloy having on a surfacethereof a non-porous aluminium oxide layer having a layer thickness inthe range of from 20 to 30 nm in the manufacture of aluminium alloysheet having a roughened surface which manufacture comprises treatingthe non-porous aluminium oxide layer with an aqueous solution of alkaliat a temperature in the range of from 35° C. to 80° C. for a period oftime sufficient to remove entirely or substantially entirely said layerfrom the alloy surface thereby leaving a roughened surface on the alloysheet.
 19. The use according to claim 18, wherein the sheet is ofAA1050A alloy.
 20. The use according to claim 18, wherein the non-porousaluminium oxide layer is treated with 2 to 5% aqueous NaOH for a periodof time in the range of from 3 to 8 seconds.
 21. A method of making agrained aluminium alloy sheet which comprises (1) subjecting a surfaceof a sheet of an aluminium alloy, to anodising conditions to form on thesaid surface an aluminium oxide barrier layer having a barrier layerthickness of from 10 to 50 nm; (2) treating the aluminium oxide layerwith an aqueous solution of alkali at a temperature of 35° C. to 80° C.for a period of time sufficient to remove entirely or substantiallyentirely said layer from the alloy surface thereby leaving a roughenedsurface on the alloy sheet, and (3) subjecting the roughened surface ofthe alloy sheet to graining.
 22. A method according to claim 21, whereinthe aluminium alloy is an alloy selected from AA1XXX and AA3XXX alloys.23. A method according to claim 21, wherein the aluminium alloy isselected from AA1050A, AA1200A and AA3103 alloys.
 24. A method accordingto claim 21, wherein in step (1) the aluminium alloy sheet is anodisedusing 20% phosphoric acid containing from 3 to 15 gl⁻¹ aluminium at atemperature in the range of from 550 to 80° C. using a current densityin the range of from 2 to 3 kAm⁻².
 25. A method according to claim 21,wherein a.c. anodising is used.
 26. A method according to claim 21,wherein d.c. anodising is used.
 27. A method according to claim 21,wherein the layer thickness of the aluminium oxide barrier layerproduced on the surface of the aluminium alloy sheet is in the range offrom 10 to 50 nm.
 28. A method according to claim 27, wherein the layerthickness of the aluminium oxide barrier layer is from 20 to 30 nm. 29.A method according to claim 21, wherein the aluminium oxide barrierlayer is non-porous.
 30. A method according to claim 21, wherein in step(2) the aqueous solution of alkali is selected from aqueous NaOH andaqueous KOH.
 31. A method according to claim 30, wherein the aqueoussolution of alkali has a concentration in the range of from 1 to 10%.32. A method according to claim 31, wherein the aqueous solution ofalkali is 2 to 5% NaOH.
 33. A method according to claim 21, wherein instep (2) the treatment is carried out for a period in the range of from3 to 20 seconds.
 34. A method according to claim 21, wherein in step (3)graining is electrograining in nitric acid.
 35. A method according toclaim 21, wherein in step (3) graining is additive graining.
 36. Amethod according to claim 35, wherein the additive graining comprisesapplying to the roughened surface of the aluminium alloy sheet obtainedin step (2) a coating selected from sol gel coatings or films thatcomprise a hydrophilic and hydrophobic layer.